202 SEVENTH REPORT—1837. ; 
place (2 N=N-+N) at the elevated temperature, and that as the 
whole cools combination again takes place; the opacity being 
in some way caused by the mixed vapours. But this decompo- 
sition is by no means probable, and if it were, the change in co- 
lour, &c. is still unintelligible, so that, in the present state of our 
knowledge, the fact remains as an interesting indication of the 
probable effect of high temperatures on the internal molecular _ 
constitution even of gaseous bodies, an effect of which future ob- 
servation may be expected to furnish us with other examples. 
49. Of Electricity.—It is not improbable, that like heat, elec- 
tricity also, to which it is in so many ways related, may have an 
influence in modifying the arrangement of the crystalline mole- 
cules, so as to cause the development of one or other of the two 
forms. 
Mr. Crosse* states, that by passing a weak current of electri- 
city through solutions of carbonate of lime he obtained rhom- 
boidal crystals of cale spar at the negative electrode, and that on 
one occasion, along with these, he obtained also very fine pris- 
matic crystals, which he took for arragonite, near to the positive _ 
pole. It would be very interesting to find this statement con- 
firmed by other observers. iq 
Influenceof 50. In an early part of thisreportwehaveseenreasonto conclude _ 
Pressure. that the cause of dimorphism acts in such a way as to alter the “ 
density of the substance, or the distance at which its crystalline 
particles are placed. It is therefore interesting to inquire how _ 
far such an alteration, induced by purely mechanical means, as 
by pressure, would affect the form so as to impart to any given 
substance the characters of adimorphous body. In so far as the 
optical properties are concerned, the experiments of Sir David 
Brewster, recorded in the Philosophical Transactions for 1830, _ 
p- 87, seem to indicate that such characters may be imparted by 
mechanical agency. He found that a mixture of white wax 
and rosin, which in mass and in ordinary circumstances exhibits " 
no doubly refracting structure, yet has that structure developed _ 
in it by simple pressure between two plates of glass. Thesame — 
philosopher has also observed that in mineral substances the 
optical phenomena are changed in intensity by subjecting them 
to mechanical pressure, in the same way as they are known to 
change when exposed to a diminishing temperature. 
These facts tend to confirm the opinion above expressed, that 
heat has no specific action in producing physical changes in 
crystalline and other bodies—that it acts merely as any other 
Sialé. ceive ae 
* Reports of the British Association, vol. v., Appendix, p. 47. 
